Touch Flashcards
What are the touch, vibration and proprioception path2ay generalizations?
Primary somatosensory neuron: axon in peripheral nerve; cell body in peripheral ganglion
- Secondary somatosensory neuron: cell body in CNS; axon crossing midline; synaptic terminal in thalamus
- Tertiary somatosensory neuron: cell body in thalamus; axon through internal capsule and corona radiata; synaptic terminal in primary somatosensory cortex
- Primary sensory neuron: axon in peripheral nerve; cell body in peripheral ganglion
- Secondary sensory neuron: cell body in CNS; axon crossing midline; ascending to thalamus
- Tertiary sensory neuron: cell body in thalamus; axon through internal capsule and corona radiata; ending in primary somatosensory cortex
What makes up the dorsal columns?
- Axons of the primary somatosensory neurons form the dorsal (posterior) columns
- They carry sensory information of touch, vibration and proprioception
Crossing of the axon…
Of the secondary somatosensry neuron in the lower medulla
What are the generalizations of the touch pathways?
Generalizations of the Touch Pathways
• Three neuron chain
• First order neuron (somatosensory receptor neuron): afferent fibers in peripheral nerve
• Second order neuron: fibers crossing the midline and terminating in the thalamus
• Third order neuron: fibers running in posterior limb of internal capsule and ending in S1 (primary somatosensory cortex)
Describe the spinal pathway for touch
Spinal Pathway for Touch: Dorsal Column / Medial Lemniscus
In the previous lecture we have focused on the dorsal column / medial lemniscus pathway up to the level of the spinal cord. The schematic drawing we use to describe the complete pathway has been introduced earlier.
We will use this schematic diagram for many of the sensory pathways we will cover in lecture, so you should familiarize yourself with this scheme. The orientation, unless marked differently, will be the same as in facing a patient (the right half of the diagram represents the left side of the patient, and vice versa).
What is the spinal pathway for touch, vibration and proprioception?
The somatosensory pathway for touch and vibration starts in the body’s surface with the dermatomes discussed in the previous lecture. The pathway for proprioception starts at the muscles. All primary sensory fibers enter the spinal cord through the dorsal roots of the spinal nerves. Within the spinal cord the fibers ascend in the dorsal column on the ipsilateral side. They synapse in the lower medulla in the nuclei of the dorsal column, i.e. the cuneate or the gracile nucleus.
The cell bodies of second order neurons of the touch pathways are located in these nuclei of the dorsal column. Fibers originating from these neurons after exiting the nucleus immediately cross the body’s midline and ascend further in the medial lemniscus, until they synapse in the ventral posterior lateral (VPL) nucleus of the thalamus.
Fibers of the third order neurons originating in the thalamus ascend through the posterior limb of the internal capsule, form part of the corona radiata and synapse in the primary somatosensory cortex (S1), which is located in the postcentral gyrus of the parietal lobe of the cerebral cortex.
The sections on the right-hand side of the Dorsal Column / Medial Lemniscus diagram below depict representative mid-sections, from top to bottom, of the midbrain, pons, medulla and spinal cord. You have to be able to localize the fiber tracts carrying sensations of touch, vibration and proprioception in these sections (either depicted schematically, as in this diagram, or as part of an MRI).
What is the trigeminal pathway of touch, vibration and proprioception?
The trigeminal pathway for touch, vibration and proprioception covers the input from cutaneous mechanoreceptors and muscle proprioceptors of the face.
Primary afferent sensory receptor neurons have their cell body in the trigeminal ganglion. The primary afferent fibers carrying touch, vibration or proprioception sensory information enter the brainstem at the pons. They synapse in the principal (chief) sensory nucleus of CN V in the upper half of the pons.
You should review all trigeminal nuclei and their location using Haines Atlas, 10th edition, Figure 8-7.
Fibers of second order neurons exit the principal nucleus of CN V and immediately cross over to the contralateral side, where they ascend in the ventral trigemino-thalamic tract. They synapse in the ventral posterior medial (VPM) nucleus of the thalamus.
Third order neurons in the VPM nucleus of the thalamus send their axons through the posterior limb of the internal capsule and then through the corona radiata. They synapse in the primary somatosensory cortex (S1).
When you look at the sections of the brainstem you realize that fibers carrying somatosensory information of touch, vibration and proprioception from the face can be found only in the upper pons and in the midbrain (close to the medial lemniscus).
What are the pathway generalizations of trugeminal touch, vibration and proprioception?
- Primary sensory neuron: axon in peripheral nerve; cell body in peripheral ganglion
- Secondary sensory neuron: cell body in CNS; axon crossing midline; ascending to thalamus
- Tertiary sensory neuron: cell body in thalamus; axon through internal capsule and corona radiata; ending in primary somatosensory cortex
Where is the primary somatosensory cortex (S1)?
Brodmann areas post central gyrus
Posterior paracentral lobule (gyrus)
how to find the posterior paracentral lobule?
- Find the cingulate sulcus and follow it to the margin between medial and lateral surfaces; the last section is called marginal sulcus (or marginal ramus of the cingulate sulcus)
- From the marginal sulcus, jump one sulcus towards frontal, and you reach the central sulcus (just a tiny indentation on the medial surface)
- Between the central sulcus and the marginal sulcus, you find the posterior paracentral lobule (or gyrus)
Discuss the topographical Organization of the Primary Somatosensory Cortex
You can compare the somatotopic map of the primary somatosensory cortex with a topographical map of a certain area. In a topographical map, areas that lie close to each other in the real geographical world (such as Manhattan and New Jersey) are depicted close to each other on the map. Areas distant to each other in the real world, such as New York and Miami, are depicted distant from each other on the map.
Similarly, on the “somatotopic map” of the somatosensory cortex, areas that are close to each other on the body’s surface, such as leg and foot, are in close proximity to each other in the somatosensory cortex. Areas distant to each other, such as foot and hand, are represented distant to each other on the somatotopic map. The picture on the right shows a few characteristic landmarks of the topographical map of the primary somatosensory cortex.
You may notice that the face does not fit well into the overall map (remember the different pathways for face and body). However, within the face area, the mapping principle is maintained, with the upper lip close to the lower lip, et
Explain two point discrimination
Our ability to distinguish two separate touch stimuli from each other varies significantly across the body’s surface.
The slide below gives you a few examples of two-point discrimination thresholds. These numbers vary between individuals, but the general tendency remains the same: low threshold (= high spatial resolution) at the fingertips; higher threshold (= lower spatial resolution) at the palm of the hand. If the distance between the two points is below the threshold, the perception of the test person is “one point”; if the distance between the two points is above threshold, the person perceives “two points”.
Several factors contribute to low two-point discrimination thresholds for touch (which is characteristic of high spatial acuity):
- High density of cutaneous mechanoreceptors
- Small receptive fields of these receptors (superficial receptors usually have smaller receptive fields, compared to receptors located deeper in the tissue)
- Low levels of convergence along the touch pathway
- Lateral inhibition (wiring mechanism that amplifies the difference between stimulus and background and improves differentiation between two stimuli close to each othe
Explain neurological examination of touch, vibration, and proprioception
When examining touch, vibration and proprioception, the examiner asks the patient to keep the eyes closed in order to avoid visual clues.
Touch can be examined together with pain, using either side of a safety pin (or more sophisticated special devices with dull and sharp ends). The examiner touches different areas of the body’s surface with one end of the safety pin, asking the patient “Is this sharp, or dull?”
Asking the patient to decide between alternatives, rather than purely affirmative statements, such as “yes, I can feel this”, allows the examiner to distinguish between touch (based on low threshold mechanoreceptor activation) and pain (based on high threshold mechanoreceptor activation) modalities and their underlying pathways.
What are the meanings of touch?
In class we will discuss neurological examination of touch, vibration and proprioception. As part of your practical clinical skills training you will be performing these tests in the small group practical sessions.
Whereas in our course notes we only use the term “touch”, with the implication of “discriminatory touch”, you might come across some additional touch-related terms, which seems to necessitate the following clarifications:
Apart from the term “touch”, some textbooks of Clinical Neuroscience use the term “light touch”, in particular when referring to neurological examination using cotton swabs or brushes. In his book “Neuroanatomy and the Neurologic Exam, CRC Press © 1994”, Terence R Anthoney points out that none of the reviewed textbooks in Clinical Neuroscience “explicitly describes the differences in the meanings of these terms”. Based on the context these terms are used in, he concludes that “some call this modality “touch”, some call it “light touch”, and some use both terms interchangeably”.
According to the same author, when investigating the use of the terms “crude touch” and “simple touch”, he states that he “found no author of a recent clinical text who uses either of them in the context of the neurological examination”.
What is vibration?
Vibration sense is often the first sensation lost in peripheral neuropathies. To test vibration sense, a relatively low-pitched tuning fork (128 Hz or 256 Hz) is placed firmly on prominent structures at the joints, for example the interphalangeal joints of a finger or toe. To be certain the patient recognizes the vibration (and not only the touch), try the tuning fork while it is still swinging or after it has been stopped.
